Historically, the containers or tanks used for storage or transport of chemicals were made of metal. These metal articles have gradually been replaced with single-layer plastics, which are lighter and are easier to use, allowing tanks to be made with complex geometries, and are not susceptible to corrosion. However, these single-layer plastic tanks no longer comply with the regulations on emissions and it is therefore necessary to develop plastic solutions with better performance in terms of barrier properties than the single-layer solutions. One route used industrially today is the multilayer plastic structure combining different materials judiciously selected in order to increase the barrier properties of said structure. The transport sector notably is constantly seeking to reduce the weight of vehicles, to lower their emissions and their energy consumption. However, these lightweight materials must meet the strictest hermeticity standards. Therefore the polymer wall of a fuel tank must have good barrier properties to limit mass transfer from the fuels to the environment. The word barrier is used in the description to describe barrier materials to essences. To perform its barrier role best, the wall of the tank is notably formed from a multilayer polymer structure, each of the layers performing a role of barrier to at least one of the compounds in the fuel.
Environmental standards are more and more stringent, for maximum reduction of emissions of hydrocarbons into the environment. Thus, the European standard Euro V, in force since September 2009, limits emission to 2.0 g per test, and a test may take up to 36 hours. The American standard PZEV (acronym for “Partial Zero Emission Vehicle”) limits the emissions to 0.35 g per test of 3 days for the whole vehicle. Consequently, in this context, it is necessary to optimize the walls of the fuel tanks, by optimizing their permeation to essences, for maximum limitation of any loss of fuel to the environment.
An essence is a mixture of aromatic and aliphatic compounds and of an oxygen-containing compound, very often alcohol. Now, polymers are not barriers to all the compounds contained in an essence and not all polymers are barriers to the same compounds contained in an essence. Thus, polyethylene for example is a barrier to alcohol, but not to toluene, which is an aromatic compound that is a constituent of the essence.
The key materials used in the barrier structures are characterized by low levels of swelling when they are brought into contact with the essences, and by reduced mobility of the small molecules diffusing in the polymer matrix. This imposes very long delays for equilibration for the industrial structures of multilayer tanks, typically of the order of a year. Moreover, experimental investigation of these phenomena requires accumulating numerous measurements. These considerations have led the applicant to perform these characterizations on the basis of much thinner films than those that are used in the walls of tanks. In fact, each time the thickness of a film is halved, its time for semi sorption/desorption is approximately divided by four and the mass flow is amplified by a factor of two. The experimental measurements are still very time-consuming since they take on average from 1 to 3 months for essence lines with a thickness of 1 mm, and at least a year for tanks with a minimum thickness of 3 mm.
In view of the extremely long time for obtaining measurements allowing validation, or otherwise, of multilayer polymer structures for making a tank wall, it became apparent that it is necessary to develop models to allow preselection of structures that are usable and comply with the standards. With such modeling it is possible to validate structures in one day, whereas the experiments take several months.
There are models for predicting permeability through multilayer structures, but these models are used in the context of investigation of permeability to simple gases, such as carbon dioxide CO2, oxygen O2, or nitrogen N2, or to water vapor. These models make it possible for example to predict the storage life of foodstuffs by estimating the oxygen permeability of a multilayer membrane surrounding the foodstuffs. However, these models do not allow prediction of the permeability of multilayer structures to an essence. In fact, an essence is a solvent mixture and the composition of the essence varies over the thickness of the multilayer structure as a function of the nature and thickness of the layer traversed. Various parameters must therefore be taken into account, for example interactions of the essence/polymer type (sorption), the resistance at the interface between two polymer layers and diffusion of the essence, and of each of its constituents, in each layer.
On the basis of laws of diffusion (Fick's Law for example) and laws of sorption (UNIQUAC model for example), defined for each monolayer, the behavior of multilayer structures combining different materials is predicted, and thus structures of tanks are validated, or otherwise, with respect to ever more stringent environmental standards.
A method is also known from the document with the title “Modeling of fuel permeation in multilayer automotive plastic fuel tanks”, published in April 2011 by Benrabah, Thibault and DiRaddo, for modeling the permeation of essences through multilayer materials, such as tanks, but the hypotheses adopted in this method are very limiting, do not apply to all of the layers of the tanks described and consequently do not allow a reliable result to be obtained.